This invention pertains generally to the assembly of microcircuitry and more specifically to the mounting, by means of a conductive epoxy bonding resin, of a plurality of miniature transistor chips on a hybrid circuit base having contact pads thereon.
The manufacture of hybrid circuits resembles, on the one hand, integrated circuit technology and, on the other, discrete component technology, hence the name hybrid. The hybrid technology is similar to the discrete component manufacturing process in that each active device or transistor chip is assembled independently onto a larger circuit base and is different from the discrete technology because the active device remains unencapsulated until the entire circuit is assembled.
Circuitry of the hybrid type resembles the integrated technology more closely in that the basic circuit is etched onto a ceramic base covered with a thin gold film. The film is approximately 300 A thick and defines most of the circuit connections that fit onto a ceramic base small enough to be encapsulated into a standard size dual in-line package. This allows compatability with integrated circuit technology while still retaining the flexibility of discrete component circuitry, a basic advantage of hybrid technology.
The small active component chips (10.times.10 mils) in each circuit are usually individually bonded with a conductive epoxy to somewhat larger gold areas (20.times.10 mils) normally called contact pads. The conductive epoxy is generally applied to the pads by epoxy dispensing machines having a long cylindrical dispensing tip for making dots of epoxy. The machines in the prior art known to be available for this type of application are those manufactured and sold by Motion Dynamics Research Inc., Mech-El Ind Inc. and Laurier Associates.
There have been some problems using these prior art machines in the assembly of the beforementioned hybrid circuits for which the invention provides a solution. One problem has been contamination of the circuitry by the dispensing tip. If the dispenser touches the circuitry, any accumulation of dust or other foreign particulate matter on the sides and edges thereof may contaminate the surface of the gold etching. It is particularly critical that this not happen since the circuit has yet to be encapsulated into a protective plastic container. The contaminants may have the undesirable effect of altering the electrical characteristics of the circuitry. Any epoxy left on the dispensing tip following each dispensing operation provides an environment conducive to such contamination.
Another problem with prior art machines has been the separation of the conductive epoxy binder into its respective components. The conductive epoxy binder is formed by the suspension of conductive particles such as gold or silver (filler) in a resin type binder (carrier). This type of suspension tends to become unbalanced or separated into its individual components of carrier and filler in response to any friction between the filler and the inner dispenser wall. Such unbalancing or separation is a function of two variables -- (1) the particulate size of the filler and (2) the diameter of the orifice through which the binder is dispensed.
If the orifice is too small or the filler particles to large, the particles will drag along the inner walls of the dispenser causing a bunching effect and a resulting unbalance in the ratio of filler to resin deposited. The bunching of the filler particles left behind eventually causes the cylindrical orifice of the dispenser to clog and thereby causes production time losses. More importantly, the unbalanced deposited epoxy is less conductive than before since it has many less conductive particles than is desirable in the amount deposited. The resulting higher resistance of the deposited epoxy is undesirable as it becomes an unpredictable circuit parameter caused by the construction and assembly. The prior art has partially solved this problem by providing epoxies with smaller particle sizes and by the use of larger dispenser orifices. However, the larger sized orifices that are needed to produce acceptable results in terms of proper binder composition are much more difficult to control as to the reproducibility in size of the epoxy dot or drops deposited.
It is extremely important that the epoxy dot deposited be reproducible as to shape and size. A dot that is of a larger area than usual will flow or be squeezed by a chip being bonded thereon onto other circuitry and cause unwanted electrical connections since the epoxy is a conductive suspension. If the area of the conductive epoxy deposited is smaller than usual, the electrical connection between the chip and the mounting pad may be of higher resistance than can be tolerated. The mechanical bond between the chip and mounting pad may also be weakened by smaller-than-designed-for dot areas.
One of the better ways to ensure reproducibility of a dot of conductive epoxy having a uniform area and size would be to regulate the gap distance between the work and the orifice tip of the dispenser. Most of the dispensing machines available have a micrometer control on the dispenser for attempting to effect a uniform gap distance, but fail to do so for a number of reasons. After the gap has been set on the micrometer dial, changes in height of the work holder may cause the dot size to vary as the dispenser is moved to different contact pads on the ceramic hybrid circuit base. The hybrid circuit base itself may change in thickness as it is quite difficult to manufacture the ceramic base uniformly over large areas. Even the pitch or slant of the work holder will cause these dots to be non-uniform across the entire work piece.